1. Field of the Invention
This invention generally relates to the field of manually operated automotive transmissions. More particularly, this invention pertains to synchronized gearshift changes in a manual transmission, especially those made in selected reverse drive.
2. Description of the Prior Art
In the operation of a manual-operated automotive transmission in which reverse drive, or any of the forward drive ratios, is engaged with the use of a sliding gear, coupler or another device for connecting a member of the selected gearset to a rotating shaft without first bringing the selected gear and shaft substantially to the same speed before they are drivably connected to each other, the driver must wait several seconds after depressing a neutral clutch to disengage an engine from the gearset before the gearshift lever can be moved to the selected position. A short interval is required to afford adequate time to dissipate, through friction and windage losses, the rotating inertia of the gearset after it is drivably disconnected from the power source If the gear selector lever is moved to the selected position before the speed of the gear and the shaft are substantially equal or before sufficient rotating energy is dissipated, the teeth of the coupler or synchronizer sleeve will strike the clutch teeth of the member to be engaged The force applied manually by the vehicle operator to the coupler or synchronizer sleeve toward the direction of their engagement with the selected gear will cause a loud clash as the tips of the sleeve teeth and those of the dog or clutch teeth are forced together but are unable to mesh because of their relative rotational speeds. The clash is objectionable to most drivers and can, if the forces applied by the operator are large enough, damage the clutch teeth or those of the sleeve.
An obvious technique to avoid this difficulty is to provide fully synchronized gear engagement. This approach has been adopted throughout the automotive industry with respect to all of the forward drive ratios including the lowest speed ratio. However, the cost and complexity to synchronize the reverse drive engagement has not been fully justified commercially, particularly in view of the limited need for reverse synchronized engagement. Generally, it can be assumed that reverse drive is selected when the vehicle is operating at low speed or when the drive wheels are fully stopped through operation of the wheel brakes. Furthermore, the delay between the time when the neutral clutch pedal is depressed to disengage the engine from the gearset and the time when the selector lever can be moved to the reverse drive position without producing clash is acceptable, though longer than the period required for this action between gear shifts in the forward drive ratios while the vehicle is accelerating or traveling at high speed.
Nonetheless, any delay after the neutral clutch pedal is depressed and before the shift lever can be moved to the reverse drive position to avoid causing clash is objectionable, particularly so in a small vehicle where high speed engines and manual transmissions are often used. Even at the idle speed of small modern engines, the gearset when driven by the engine turns at a high speed, has inherently high inertia and requires longer periods of delay before the reverse drive or any unsynchronized gear shift can be completed.